Recent Advances in Hydrodeoxygenation of Biomass-Derived Oxygenates over Heterogeneous Catalysts
Hydrodeoxygenation (HDO) using heterogeneous catalysts has received considerable attention as a way of converting biomass-derived oxygenates into renewable fuels and chemicals. HDO involves combinations of different reactions such as hydrogenation, hydrogenolysis, decarbonylation, and dehydration. The reactions occur at different catalytic sites typically with heterogeneous catalysts (e.g., metal, acid, and bifunctional sites), making HDO reaction complex. Thus, the selection of active site type is critical when designing effective heterogeneous catalysts for the HDO process. Catalyst stability is a major issue of designing HDO catalysts. Traditional catalysts are not stable under HDO conditions (i.e., a high partial pressure of water). Metal particles are leached and/or sintered. Solid-acid support undergoes phase transformation and surface area loss. Condensation or polymerization of C=C or C=O bond within biomass and its derived oxygenates is easy to conduct under high temperature in the HDO process, causing carbon deposition on the catalyst. The poor catalyst stability is a major challenge that needs to be overcome for innovation in HDO technologies. Therefore, herein, we focus on providing insight into how to design effective catalysts for HDO. First, the roles of different catalytic sites in HDO and strategies to stabilize these active sites are discussed. Current achievements in HDO of different biomass feedstocks (e.g., model compounds and bio-oil) over various heterogeneous catalysts are highlighted. Recent developments in electrochemical HDO processes, which do not require gaseous hydrogen, are discussed. Finally, it is discussed the points to be considered for further research into HDO over heterogeneous catalysts and the pending challenges.